This invention relates generally to perceiving acceleration upon an object. More specifically the invention relates to the devices used for making such perceptions, known as accelerometers and particularly an optical accelerometer created through the technology known as micro-electro-mechanical systems orxe2x80x9cMEMSxe2x80x9d. With yet greater exactness, the invention relates to adjusting the sensitivity of such an accelerometer by selectively and independently positioning interferometers mirrors utilized in the invention.
Micro-electro-mechanical systems use microelectronic processing techniques wherein mechanical devices arc reduced to the scale of microelectronics. These processing techniques enable the integration of both mechanical and electrical components onto a single chip, typically made of silicon. Prior to MEMS, accelerometer components were for the most part manufactured separately. These components were then assembled together in a process that could easily be complex and expensive.
Current MEMS accelerometer designs have numerous advantages over their conventional counterparts. The MEMS accelerometers are of small size, light weight and low cost. Their sensitivity, however, has fallen largely in the low performance regime. Because of their relative low sensitivity and cost, current MEMS accelerometers have been used primarily in the automobile industry as collision airbag sensors and for other low sensitivity applications. Although the collision airbag sensor market is significant, it is but a small fraction of the potential market for low cost ultra-sensitive MEMS accelerometers.
Existing MEMS accelerometer technology is based upon either capacitive or piezo-based designs. State of the art MEMS capacitive accelerometers measure the charge on a capacitor to detect small movements of a proof mass attached to a spring. However, in order to detect sub milliG (1 G=9.8 m/s2) perturbation forces with this technique, elaborate amplification circuitry capable of measuring on the order of nanovolt changes in potential is necessary. For example, typical steady state capacitance values for MEMS accelerometers are in the 100 fFarad range, where 1 fxe2x88x9210xe2x88x9215. Furthermore, a 1 G accelerating force results in a minute change in capacitance, on the order of 100 aF where a=10xe2x88x9218. This is equivalent to sensing a change of 625 electrons across the plates of a capacitor at an applied bias of 1 volt. Alternatively stated, this is commensurate with detecting the presence/absence of approximately 1 out of every 1000 electrons. To amplify this small change in capacitance extremely sensitive circuitry is required to translate the capacitance variations into a detectable voltage output signal. Even with the addition of low noise amplification circuitry, these MEMS accelerometers do not have the sensitivity required for many potential applications.
Piezoelectric or piezoresistive materials produce either a potential difference or a change in resistance when an external pressure/force is applied. This property lends itself to accelerometer designs. A shortcoming of piezoelectric or piezoresistive materials is that they are also pyroresistive, meaning that they change resistance with temperature. High sensitivity piezo-based accelerometers are therefore difficult to maintain. In addition, the resistance or change in potential of such accelerometers is usually extracted from a large resistor fabricated in the material. This large resistance leads to increased noise, e.g. resistive noise/Johnson noise. These problems are significant for piezo-based accelerometers. More commonly used accelerometers therefore use the capacitive methodxe2x80x94which also suffers from noise but not as severely.
To realize the full potential of MEMS accelerometers, a significant improvement in sensitivity is required. Ideally, this improvement will minimize accelerometer inherent noise. Possible applications of such low cost, light weight, ultra-sensitive MEMS accelerometers include bio-mechanics, seismology, condition monitoring of machines and structures, and robotics. In addition, an ultra-sensitive MEMS accelerometer would dramatically improve the accuracy of guidance, navigation, and global positioning systems (GPS) that require sensitivity not on the order of the 1 G scale but rather the on the order of the xcexcG scale or better.
The invention has structural similarities to an optical switch and amplifier described in the article titled: xe2x80x9cMicromechanical Optoelectric Switch and Amplifier (MIMOSA)xe2x80x9d by R. Waters et al, IEEE Journal of Selected Topics in Quantum Electronics, 5, 33 (January/February 1999) incorporated by reference herein.
The invention is an improvement upon an accelerometer based upon the monolithic integration of a Fabry-Perot interferometer and a p+n silicon photodiode. The transmission of light through a Fabry-Perot etalon is exponentially sensitive to small displacements in the position of a movable mirror due to changes in an applied accelerating force. The photosensor converts this displacement to an electrical signal as well as provides for additional amplification. Because both the Fabry-Perot modulator and photodiode are monolithically integrated on a silicon substrate, the combination is compact and has minimal parasitic elements, thereby reducing the accelerometer""s noise level and increasing its signal-to-noise ratio (SNR).
The sensitivity of the invention is user-controlled based upon any one or a combination of factors: adjusting the length between the mirrors of the Fabry-Perot etalon; adjusting the power of the light projected through the mirrors to the photodiode; and activating and deactivating the light at a selected frequency to minimize 1/f inherent system noise in the response of the accelerometer.
In the present design, the length between the mirrors of the Fabry-Perot etalon is adjustable by a mechanism lying outside of the optical path of the photodiode and that does not utilizing the photodiode itself as a conductor to aid in the adjustment of the mirrors.
The MEMS accelerometer of the invention is calculated to be capable of producing 1 V/G without the use of amplification circuitry. It is estimated that when amplification circuitry is used with the novel MEMS accelerometer of the invention, it will be more than three orders of magnitude more sensitive than present MEMS accelerometers using amplification circuitry. This implies that the xcexcG sensitivity needed for navigation and GPS applications is obtainable if voltage levels on the order of 1 xcexcV are detectable.
In opposition to prior art designs, the invention uses a light source rather than capacitive or piezo-based techniques for sensing acceleration. The advantages of this include use of a small wavelength of light for accurately measuring the movement of a suspended inertial mass and utilizing the wave nature of light for creating an exponentially sensitive accelerometer that is more than three orders of magnitude more sensitive than the previous art.
An object of this invention is to provide an accelerometer of high sensitivity.
Another object of the invention is to provide an accelerometer of high sensitivity in which inherent (1/frequency) noise is minimized.
A further object of this invention is to provide an optical accelerometer of high sensitivity.
Still another object of this invention is to provide an optical accelerometer in which light power is varied to adjust the accelerometer""s sensitivity.
Still yet another object of this invention is to provide an optical accelerometer in which light power is varied to adjust the accelerometer""s sensitivity by decreasing system inherent noise.
Still a further object of this invention is to provide an optical accelerometer in which light is selectively pulsed to adjust the accelerometer""s sensitivity.
Still yet a further object of this invention is to provide an optical accelerometer that includes a Fabry-Perot etalon in which the distance between the etalon""s mirrors is adjusted to adjust the accelerometer""s sensitivity.
Yet another object of the invention is to provide an optical accelerometer that includes a Fabry-Perot etalon in which the distance between the etalon""s mirrors is adjusted to adjust the accelerometer""s sensitivity and in which this adjustment is done by a mechanism lying outside the light path of the accelerometer and that does not employ the light sensing mechanism to effectuate the mirror adjustment.
Yet still a further object of this invention is to provide an optical accelerometer of high sensitivity that is fabricated through micro-electro-mechanical system (MEMS) processing.
Other objects, advantages and new features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanied drawings.